Field of the Invention
The present invention relates to a polarizing film and a method for manufacture thereof. The polarizing film may be used alone or as a part of a laminated optical film to form image display devices such as liquid crystal displays (LCDs), organic electroluminescent (EL) displays, cathode ray tubes (CRTs), and plasma display panels (PDPs).
Description of the Related Art
The liquid crystal display market has experienced rapid growth in many applications such as clocks, cellular phones, personal digital assistants (PDAs), notebook PCs, PC monitors, DVD players, and TVs. Liquid crystal display devices use liquid crystal switching to visualize the polarization state, and based on the display principle, they use polarizers. Particularly in TV applications and so on, higher brightness, higher contrast, and wider viewing angle are required, and polarizing films are also required to have higher transmittance, higher degree of polarization, and higher color reproducibility.
For example, iodine polarizers made of stretched polyvinyl alcohol (hereinafter, also simply referred to as “PVA”) to which iodine is adsorbed have high transmittance and high degree of polarization. Therefore, they are most popular polarizers widely used. A polarizing film commonly used includes a polarizer and transparent protective films bonded to both sides of the polarizer with a solution of a polyvinyl alcohol-based material in water, what is called an aqueous adhesive (Patent Documents 1 and 2 listed below). Triacetylcellulose or the like has high water-vapor permeability and therefore is used for transparent protective films.
A polarizing film can be produced using an aqueous adhesive such as a polyvinyl alcohol-based adhesive. In this case (what is called wet lamination), a drying step is necessary after a polarizer and a transparent protective film are bonded together. To increase polarizing film productivity, it is preferable to shorten the time required for such a drying step or to use an alternative bonding method with no need for any drying step.
Also when an aqueous adhesive is used, a polarizer needs to have a relatively high moisture content so that the adhesive can have high tackiness to the polarizer (a common polarizer has a moisture content of about 30%). Otherwise, the adhesive cannot provide good tackiness in the resulting polarizing film. Unfortunately, the polarizing film obtained in this way also has a problem such as a significant dimensional change at high temperature or high temperature and high humidity or low optical properties. To reduce such a dimensional change, a low-moisture-content polarizer or a low-water-vapor-permeability transparent protective film may be used. However, if such a polarizer and such a transparent protective film are bonded with an aqueous adhesive, drying efficiency or polarizing properties can degrade, or an appearance defect can occur, which can make it impossible to obtain practically useful polarizing films.
In recent years, as the screen size of image display devices (particularly typified by TVs) has increased, an increase in the size of polarizing films has also become very important in terms of productivity and cost (an increase in the yield or the number of available pieces). Unfortunately, polarizing films produced with the aqueous adhesive have the following problem. They can be dimensionally changed by heat from a backlight. The dimensional change can cause unevenness, so that a phenomenon in which a white part is visible against black background displayed on the whole of a screen, what is called light leakage (unevenness), can be significant.
To solve the problem with wet lamination, active energy ray-curable adhesives are proposed which contain no water or organic solvent. For example, Patent Document 3 listed below discloses an active energy ray-curable adhesive containing (A) a polar group-containing, radically polymerizable compound with a molecular weight of 1,000 or less, (B) a polar group-free, radically polymerizable compound with a molecular weight of 1,000 or less, and (D) a photopolymerization initiator. Unfortunately, this adhesive tends to have low tackiness to polarizing films because the combination of radically polymerizable compounds (monomers) as components of this adhesive is designed to improve tackiness especially to norbornene resin films.
Patent Document 4 listed below discloses an active energy ray-curable adhesive including, as essential components, a photopolymerization initiator with a molar absorption coefficient of 400 or more at a wavelength of 360 to 450 nm and an ultraviolet-curable compound. Unfortunately, when used on polarizing films, this adhesive tends to have low tackiness to polarizing films because the combination of monomers as components of this adhesive is designed to prevent warpage or deformation mainly during bonding of optical discs or the like.
Patent Document 5 listed below discloses an active energy ray-curable adhesive containing (A) a (meth)acrylic compound having two or more (meth)acryloyl groups in the molecule, (B) a (meth)acrylic compound having a hydroxyl group and only one polymerizable double bond in the molecule, and (C) a phenol ethylene oxide-modified acrylate or a nonylphenol ethylene oxide-modified acrylate based on 100 parts by weight of the total amount of the (meth)acrylic compounds. Unfortunately, in the combination of monomers as components of this adhesive, the monomers have relatively low compatibility with one another, which can cause phase separation and a risk of a reduction in the transparency of the adhesive layer. This adhesive also has a risk of reducing durability such as crack resistance because it is designed to improve tackiness by softening (reducing the Tg of) a cured product (the adhesive layer). Crack resistance can be evaluated by thermal shock test (heat shock test).
The inventors have developed a radically polymerizable, active energy ray-curable adhesive by using an N-substituted amide monomer as a curable component (Patent Documents 6 and 7 listed below). This adhesive exhibits high durability in a severe environment at high humidity and high temperature. Now, however, the market is demanding adhesives capable of providing better tackiness and/or higher water resistance.
Patent Document 8 listed below describes a method of manufacturing a polarizing plate, which includes boding an ultraviolet non-transmitting protective film (A) to one side of a polarizer with an ultraviolet-curable adhesive interposed therebetween, bonding an ultraviolet transmitting protective film (B) to the other side of the polarizer with an ultraviolet-curable adhesive interposed therebetween, then using ultraviolet rays to carry out irradiation by way of the protective film (B) to simultaneously cure the ultraviolet-curable adhesives placed on the front and back sides of the polarizer. This manufacturing method is characterized by carrying out irradiation of ultraviolet rays by way of one side (one-stage UV irradiation) to simultaneously cure the ultraviolet-curable adhesives placed on the front and back sides of the polarizer so that high cost-effectiveness can be achieved.